Fix RichModelSummary model size display

[AsherJingkongChen]

What does this PR do?

RichModelSummary incorrectly passed model_size (already in MB) to get_human_readable_count(), which converts numbers to K/M/G format. This resulted in misleading output like "5.5 K" instead of the actual size "5,500.000".

Now uses get_formatted_model_size() for consistency with ModelSummary.

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

---

📚 Documentation preview 📚: https://pytorch-lightning--21467.org.readthedocs.build/en/21467/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (affee28) to head (0e06c06).
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (affee28) and HEAD (0e06c06). Click for more details.

HEAD has 132 uploads less than BASE

Flag	BASE (affee28)	HEAD (0e06c06)
cpu	66	33
python	6	3
lightning_fabric	18	0
pytest	33	0
python3.12	18	9
lightning	30	15
python3.11	12	6
python3.10	6	3
python3.12.7	18	9
python3.13	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21467     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         270      267      -3     
  Lines       24059    24005     -54     
=========================================
- Hits        20855    18956   -1899     
- Misses       3204     5049   +1845     

[bhimrazy]

LGTM